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.Dd $Mdocdate: September 12 2023 $
.Dt EVP_CHACHA20 3
.Os
.Sh NAME
.Nm EVP_chacha20 ,
.Nm EVP_chacha20_poly1305
.Nd ChaCha20 stream cipher for EVP
.Sh SYNOPSIS
.In openssl/evp.h
.Ft const EVP_CIPHER *
.Fn EVP_chacha20 void
.Ft const EVP_CIPHER *
.Fn EVP_chacha20_poly1305 void
.Sh DESCRIPTION
.Fn EVP_chacha20
provides the ChaCha20 stream cipher in the EVP framework.
.Xr EVP_EncryptInit_ex 3 ,
.Xr EVP_DecryptInit_ex 3 ,
and
.Xr EVP_CipherInit_ex 3
take a
.Fa key
argument of 32 bytes = 256 bits and an
.Fa iv
argument of 16 bytes = 128 bits, internally using
.Xr ChaCha_set_key 3
and
.Xr ChaCha_set_iv 3 .
The lower 8 bytes = 64 bits of
.Fa iv
are used as counter and the remaining 8 bytes are used as
the initialization vector of
.Xr ChaCha_set_iv 3 .
.Xr EVP_EncryptUpdate 3 ,
.Xr EVP_EncryptFinal_ex 3 ,
.Xr EVP_DecryptUpdate 3 ,
and
.Xr EVP_DecryptFinal_ex 3
internally use
.Xr ChaCha 3
to perform encryption and decryption.
.Xr EVP_CIPHER_CTX_ctrl 3
always fails for
.Fa ctx
objects created from
.Fn EVP_chacha20 .
.Pp
.Fn EVP_chacha20_poly1305
provides authenticated encryption with ChaCha20-Poly1305.
Unless compatibility with other implementations
like OpenSSL or BoringSSL is required, using
.Xr EVP_AEAD_CTX_init 3
with
.Xr EVP_aead_chacha20_poly1305 3
is recommended instead because the code then becomes transparent
to the AEAD cipher used, more flexible, and less error prone.
.Pp
With
.Fn EVP_chacha20_poly1305 ,
.Xr EVP_EncryptInit_ex 3 ,
.Xr EVP_DecryptInit_ex 3 ,
and
.Xr EVP_CipherInit_ex 3
take a
.Fa key
argument of 32 bytes = 256 bits and an
.Fa iv
argument of 12 bytes = 96 bits.
This supports additional authenticated data (AAD) and produces a 128-bit
authentication tag.
The constant
.Dv EVP_CHACHAPOLY_TLS_TAG_LEN
specifies the length of the authentication tag in bytes and has a value of 16.
.Pp
The following
.Fa type
arguments are supported for
.Xr EVP_CIPHER_CTX_ctrl 3 :
.Bl -tag -width Ds
.It Dv EVP_CTRL_AEAD_GET_TAG
Copy the number of bytes indicated by the
.Fa arg
argument from the tag to the location indicated by the
.Fa ptr
argument;
to be called after
.Xr EVP_EncryptFinal_ex 3 .
This control operation fails if the
.Fa ctx
is not configured for encryption or if
.Fa arg
is less than 1 or greater than 16.
.It Dv EVP_CTRL_AEAD_SET_TAG
Copy the number of bytes indicated by the
.Fa arg
argument from the location indicated by the
.Fa ptr
argument and designate them as the expected tag length and tag,
causing subsequent
.Xr EVP_DecryptFinal_ex 3
to fail if the tag calculated during decryption does not match.
It is strongly recommended to specify
.Fa arg
as exactly 16.
Otherwise, only the initial part of the tag may be compared
and mismatches near the end of the tag may get silently ignored.
This control operation fails if the
.Fa ctx
is configured for encryption or if
.Fa arg
is less than 1 or greater than 16.
If the
.Fa ptr
argument is a
.Dv NULL
pointer, this control operation succeeds without having any effect.
.It EVP_CTRL_AEAD_SET_IV_FIXED
Set the initialization vector by reading the 12 bytes pointed to by the
.Fa ptr
argument, independently of
.Xr EVP_EncryptInit_ex 3 ,
.Xr EVP_DecryptInit_ex 3 ,
and
.Xr EVP_CipherInit_ex 3 .
This control operation fails if the
.Fa arg
argument is not exactly 12.
.It Dv EVP_CTRL_AEAD_SET_IVLEN
Instruct subsequent
.Xr EVP_EncryptInit_ex 3 ,
.Xr EVP_DecryptInit_ex 3 ,
or
.Xr EVP_CipherInit_ex 3
to expect an
.Fa iv
argument shorter than the default of 12 bytes; the
.Fa arg
argument specifies the number of bytes to be used.
The initialization functions will only read
the specified smaller number of bytes from
.Fa iv
and internally zero-pad them on the left.
Using this is not recommended because it is likely more fragile
and less often tested than the equivalent method of simply providing
a full-sized
.Fa iv .
This control operation fails if
.Fa arg
is less than 1 or greater than 16.
.It Dv EVP_CTRL_INIT
Set the length of the initialization vector to the default value
of 12 bytes and clear the Poly1305 internal state.
The application program usually does not need to invoke this control
operation manually because it is automatically called internally by
.Xr EVP_EncryptInit_ex 3 ,
.Xr EVP_DecryptInit_ex 3 ,
and
.Xr EVP_CipherInit_ex 3 .
.El
.Sh RETURN VALUES
.Fn EVP_chacha20
and
.Fn EVP_chacha20_poly1305
return pointers to static
.Vt EVP_CIPHER
objects that contain the implementations of the symmetric cipher.
.Pp
If
.Fa ctx
was created from
.Fn EVP_chacha20
or
.Fn EVP_chacha20_poly1305 ,
.Xr EVP_CIPHER_CTX_ctrl 3
returns 1 for success or 0 for failure.
.Sh SEE ALSO
.Xr ChaCha 3 ,
.Xr evp 3 ,
.Xr EVP_aead_chacha20_poly1305 3 ,
.Xr EVP_CIPHER_meth_new 3 ,
.Xr EVP_EncryptInit 3
.Sh STANDARDS
.Rs
.%A A. Langley
.%A W. Chang
.%A N. Mavrogiannopoulos
.%A J. Strombergson
.%A S. Josefsson
.%D June 2016
.%R RFC 7905
.%T ChaCha20-Poly1305 Cipher Suites for Transport Layer Security (TLS)
.Re
.Sh HISTORY
.Fn EVP_chacha20
first appeared in
.Ox 5.6 .
.Pp
.Fn EVP_chacha20_poly1305
first appeared in OpenSSL 1.1.0
.\" OpenSSL commit bd989745 Dec 9 21:30:56 2015 +0100 Andy Polyakov
and has been available since
.Ox 7.2 .
.Sh CAVEATS
The original publications and code by
.An Adam Langley
used a modified AEAD construction that is incompatible with the common
style used by AEAD in TLS and incompatible with RFC 7905:
.Pp
.Rs
.%A A. Langley
.%A W. Chang
.%D November 2013
.%R draft-agl-tls-chacha20poly1305-04
.%T ChaCha20 and Poly1305 based Cipher Suites for TLS
.Re
.Pp
.Rs
.%A Y. Nir
.%A A. Langley
.%D May 2018
.%R RFC 8439
.%T ChaCha20 and Poly1305 for IETF Protocols
.Re
.Pp
In particular, the original version used a nonce of 8 instead of 12 bytes.
